Final answer:
The trans-isomer of 1,2-dichloroethene has a higher equilibrium vapor pressure than the cis-isomer at 20 degrees Celsius because the trans-isomer only exhibits London dispersion forces, while the cis-isomer also features stronger dipole-dipole interactions due to its polar nature.
Step-by-step explanation:
The question is asking which of the two isomers of 1,2-dichloroethene has a higher equilibrium vapor pressure at 20 degrees Celsius. Vapor pressure is related to intermolecular forces (IMFs) within a substance: the stronger the IMFs, the lower the vapor pressure because fewer molecules can escape to the gas phase. Cis-trans isomerism or geometric isomerism in molecules like 1,2-dichloroethene gives rise to isomers with different physical properties because their molecular shapes affect their polarity and therefore their IMFs.
The correct answer is D) The trans-isomer, because it has only London dispersion forces, whereas the cis-isomer also has dipole-dipole interactions. In the case of 1,2-dichloroethene, dipole-dipole interactions are present in the cis-isomer due to the polar nature arising from the chlorine atoms being on the same side of the double bond. This makes the cis-isomer's IMFs stronger and thus its vapor pressure lower compared to the trans-isomer, which has its dipole moments canceled out due to the opposing positions of the chlorine atoms.